1. Field of the Invention
The present invention relates to a rolling bearing having a very long life and, more particularly, to a rolling bearing having a very long life even under the severe conditions of high bearing loads, the presence of a deflection in a bearing housing, defective lubrication, etc.
2. Description of the Related Art
Recently, a situation involving the use of a bearing has become severe.
For example, the diameter of a minor diameter side working roll of a rolling mill described in Examined Japanese patent application publication No. SHO 51-47421 was reduced in order to provide a good profile in a produced steel sheet. Accordingly, the number of revolutions of the minor diameter side working roll was increased in order to roll steel material of the same weight as by a conventional rolling mill. In addition, since the diameter of a working roll 1 (see FIG. 6) of a rolling mill designed for low-temperature, high-pressure rolling or high profile control and described in Unexamined Japanese utility model application publication No. SHO 62-131705 has been more reduced, a rolling bearing used with the working roll 1 is smaller and receives a higher bearing load, and the number of revolutions of the working roll 1 is increased. In addition, since the working roll trunk receives a horizontal bending force, deflections in the working roll necks are increased. Thus, a long life rolling bearing even under such severe conditions has been desired.
Increasing the rolling contact fatigue resistance of the rolling bearing is important in increasing the life of the rolling bearing. It is known that the rolling contact fatigue resistance of the rolling bearing depends largely on an Al.sub.2 O.sub.3 inclusion or an Al.sub.2 O.sub.3 -base inclusion in the raw steel for the rolling bearing. The Al.sub.2 O.sub.3 or Al.sub.2 O.sub.3 -base inclusion present in raw steel for rolling bearings provides an origin for a fatigue failure of a rolling bearing which receives a repeated stress. Therefore, the prior art has intended to control the content of a large Al.sub.2 O.sub.3 or Al.sub.2 O.sub.3 -base inclusion to be 1.5 ppm or less in order to increase the rolling fatigue life of a rolling bearing as described in Unexamined Japanese patent application publication No. SHO 63-62847.
However, a study by the present inventors taught that simply controlling the upper limit of the content of the large Al.sub.2 O.sub.3 or Al.sub.2 O.sub.3 -base inclusion insufficiently provided a long life rolling bearing; but controlling an allowable particle size of large Al.sub.2 O.sub.3 or Al.sub.2 O.sub.3 -base inclusion particles and an allowable number thereof was important in increasing the rolling fatigue resistance of a rolling bearing.